Servo follow-up control



Feb. 1, 1949.

c. M. GILBERT SERVO FOLLOW-UP CONTROL 2 Sheets- Sheet 1 Filed Nov. 13,1946 inventor CLARKE M. GlLBERT. k attorney Feb. I, 1949. c. M. GILBERT2,460,533

SERVO FOLLOW-UP CONTROL Filed Nov. 13, 1946 2 Sheets-$heet 2 Fig.2A 3A715 75 px l J4. FfZ

H925 Fig. 3B

Fig.2C Fig-30 Snnentor CLARKE M. GILBERT.

Patented Feb. 1 1949 ITED STATES PATENT oFElcs SERVO FOLLOW-UP CONTROLClarke M. Gilbert. Chappaqua, N. Y., minor to General Precisionlaboratory Incorporated, a corporation of New York Application November13, 1948, Serial No. 709,658

v 11 Claims. 1

My invention relates to a servo control wherein the output shaft iscaused to follow the rotation of an input shaft with a high degree ofaccuracy and at the same time to deliver a substantial amount of power.

Servo controls are extensively used and not infrequently a single systemsuch as fire control systems, analyzer systems, training apparatus andthe like, use literally hundreds of such devices. that such devices bemade as small as possible and he composed of as few circuit componentspossible.

Que oi the culects of any invention'thereilore, is to provide servocontrol which includes minimum number of circuit elements but whichnevertheless is positive and accurate in opera tion.

Another object of my invention is to provide i sell relay to control therotation of a motor.

"l mother object of my invention is the pro= vision of delay means, suchthat the effective firctlon of a single thyratron is caused to exi everportion of the negative halt cycle, us accomplishing with a singlethyratron what has heret "e necessitated the use of two thyrotronsconnected in opposition.

A still further object of my invention resides in the use of a systempowered solely by alterhating current potential, thereby eliminating thenecessity of filter and rectifier circuits.

These and other objects and advantages of my invention will be apparentfrom the following specification when taken with the accompany= ingdrawings in which:

Figure l is a circuit diagram illustrating the preferred embodiment ofmy invention and Figures 2A, 2B, 20, 3A, 3B and 3C are curvesillustrating the characteristic action of the circuit Figure 1.

Referring to Figure 1 an input shaft lU- is connected to the rotor 9 ofa synchro generator Ii. The rotor 8 is connected to the power source 68and the generator H has the usual stator windings l2, l3 and H which areconnected in parallel to the stator windings l8, I1 and i8 of thesynchro transformer It. The rotor l8 of the it is extremely advantageoustherefore,

means for eflectiug servo control through the edium of but singlethyratron operating 2 syuchro transformer ll is mechanically connectedto the output shaft 50.

In the operation of synchro transmission systems such as depicted by thecircuit consisting of the generator ii and the transformer l5, when therotor is is caused to assume an angle of rotation different from that oithe rotor 9 a potential having a phase and amplitude which is a functionof the diilerence in angular positions of the rotor 9 and the rotor isis generated in the rotor it. in the present invention this potential,which is here termed an error voltage, is impressed on single thyratrouthe output at which through the action oi positioning epparm tus is usedto so rotatethe rotor is as to clecrease the error in angular p sitionbetween the rotor and rotor it hence the input shalt it and output shaftprovide a means for properly adjusting the system at zero errorconditions a hiss tired in phase and amplitude is impressed on the inputor? the thyratron ill by means or the potentiom= eter and condenser illconnected across the filament lit).

The output of the thyratroh is connected through relay coil of a relayindicated genorally by dotted lines St to the secondary t l of the powertransformer ill. Likewise connected in the output of the thyretron andassociated with the relay coil 20 is delay network consisting ofcondenser and potentiometer 2'21 for the purpose more fully hereinafterexplained. the relay ill comprises the aforementioned relay coil as anda vibrating reed armature 34 nor mally biased to contact 32 out operablewhen sufilclent current flows through the relay coil 29. as forexample-6 ma, to engage contact 33.

A two phase motor it mechanically connected to the output shaft Ellthrough a suitable gear reduction means #8, has held windings 42 and t3the opposite ends of which are connected to the respective contacts 33and 32. 'lhese windings are connected together at their other ends andto one side of the source 88 through leads and 66. The vibrating reed 34of the relay 3| is connected to the opposite side of said source 68, 62and 63. Limiting resistors 38 and 38 and shunt condensers 35 and 38 areprovided in the relay circuit to prevent sparking of the contacts.

To provide an effectivemeans for imparting torque to the motor 43 andfor enabling the direction of the applied torque to be reversed, acondenser It is connected in shunt with the contacts 32 and 33 and thefield windings 82 and 33 of the motor dd.

Suitable gearing 38 is provided between the motor Ml and the load at anda damping device 4? is also connected to the motor shaft as an anti-huntdevice.

The operation of the device may be described as follows: when vibratingreed 34 engages the contact 32 a potential is supplied from the source63 by a circuit consisting of leads'62 and 83, reed 33, contact 32,resistance 39, winding 33 of the motor it leads 6% and 33 back to thepower source 38.

The potential impressed on winding 62 however, is supplied through acircuit consisting of leads 62 and 63, reed 36 and contact 32, resistor39, condenser M and winding d2, thence through return leads 65 and at tosource 58.

It will .be seen therefore, that potential is applied directly acrosswinding- 63 but is applied to winding 42 and condenser it in series. Thepotential across winding 32 therefore leads that impressed across 63 andthe motor is caused to turn in a clockwise direction.

On the other hand when the current through winding 29 is above thepredetermined minimum and the reed 33 is caused to engage contact :33potential is supplied directly across winding 62 through leads 62 and'63, reed 34, contact 33, resistance 38, winding 32 and return throughlead 65. The same potential however, is applied to winding 43 throughthe condenser M by a path consisting of leads 62 and 63, reed 35,contact 33, resistance 33, condenser 4i, winding 43 and return throughleads 65 and 63. Under these circumstances, the potential applied acrosswinding 43 now leads the potential applied across winding 42 and themotor is caused to revolve in a counterclockwise direction.

If thevibrating reed 38 is actuated at a rapid rate, for example-45ocycles per second, the motor will tend to revolve first in one directionand then in another, but inasmuch as it has some inertia no resultantmotion will take place, providing the reed 33 engages contact 32 for thesame average length of time over a period of several cycles as the reed35 engages contact 33. This is the desired operation of the device atzero setting, that is, when the input shaft to has th same angularposition as the shaft 50. I

If on the other hand the reed 34 engages either contact 32 or 33 for agreater average length of time there will be a resulting torque ineither a clockwise or counterclockwise direction depending on whichcontact is engaged for the greater length of time. That is to say,considering t to represent the time of engagement of contact 33 and t2to represent the time of engagement of contact 32, then when his greaterthan tz there is a counterclockwise torque produced while when 222 isgreater than t1 there is a clockwise torque produced and where t1 equals152 no torque is produced.

.Figs. 2A, 2B and 2C indicate the operation under zero conditions whenthe input shaft l and output shaft have the same angular position and noerrorlhas been introduced into the system and the curves of Figs. 3A, 3Band 30 indicate the zero error conditions the potential applied to the 7input circuit of the thyratron 2l' which is the algebraic sum of thepotentials of the phase biasing network comprising condenser 57 andpotentiometer 56 and potential developed in the rotor I9 is caused tofollow the curve E? by means of suitable adjustment of the phasenetwork. Under these conditions, the grid potential as indicated bycurve 12 reaches the firing potential as represented by curve ll whenthe plate potential ill is at its peak value. The thyratron fit isthereby caused to fire during a quarter cycle as indicated by the solidline curve 13 of Fig. 213.

If no other means were provided, current would flow in the thyratronoutput circuit and in coil 29 of the relay 3! only during one quarter ofa cycle and hence reed 33 Would be caused to engage contact 33 onlyduring this quarter cycle and would engage contact 33 during theremaining three quarters of the cycle. The reed 38 would thereforeengage contact 32 for a period approximately 3 times as long as itengages contact 33.

To provide the criterion of equal lengths of time of contact under zeroconditions, the delay network consisting of condenser 28 andpotentiometer 2'! is provided. This delay network is so adjusted thatwhen the thyratron 28 is caused to fire the condenser 28 is chargedrapidly through the left hand end of potentiometer 27 and when thethryatron is no longer in firing condition the condenser 28 dischargesrelatively slowly through the right hand end of potentiometer 27 andrelay coil 29 maintaining the current in this coil above its thresholdoperating current condition as represented by dotted line Hi of Fig. 2B.A curve representative of the current flowing through the relay coil 29is indicated by the dash dot curve 15 of Fig. 2B and it will be seenthat this current is maintained above the threshold operating current ofthe relay coil 29 for a full half cycle. This results in an operation ofthe relay contacts as indicated diagrammatically by the graph of Fig. 2Cwherein reed 33 engages contact 33 for a time i1 and for the remainderof the cycle engages contact 32 for an equal time 132. Under suchcircumstances as heretofore indicated, no resultant motion of the motor64 takes place.

Consider now a condition wherein the output shaft 50 has a differentangular relationship with the input shaft l0. Under these conditions,the potential applied to the input of the thyratron 2| which consists ofthe voltage generated in the winding I9 of the synchro l5 and the biasvoltage of the phase network 53 and 57 is such that the family of curvesof Fig. 3A are produced. As in Fig. 2A the solid curve it represents thepotential applied to the output of thyratron 2| and the curve Hrepresents the firing potential. The increase in voltage generated inthe winding I9 causes the thyratron to strike at a point prior to thepeak potential applied to the plate current as indicated by the dottedcurve 15.

Referring now to the curves of Fig. 33 it will be seen that thethyratron is operative for a asoa oas greater proportion of the time.Again by reason of the delay network consisting of condenser 20 andpotentiometer 21 the current in-the relay coil 20 is maintained abovethe threshold current for a length of time after the thyratron hasceased to fire and this current follows the dash dot curve 18. Underthese circumstances the reed 34 is caused to engage contact 33 for atime t1 which is greater than the time t: which i the reed 34 engagescontact 32. There is therefore as heretofore described, a torque on themotor which causes it to revolve in a counterclockwise direction at aspeed depending on the torque applied thereby causing the output shaft50 to assume the same angular relationship of the input shaft l and theerror to'be corrected. It will be seen therefore that ii the outputshaft 50 ends to assume a different phase relationship from that of theinput shaft it, an error voltage will be impressed on the servomechanism which through action of the thyratron 2i and relay II willcause the motor 44 to correct such error in phase relationship.

While there has been described what is considered the preferredembodiment of the invention, it will be obvious to those skilled in theart to which it appertains that various modifications may be made withinthe spirit and scope of the said thyratron control means. relay meanscon" trolled by the thyratron control moans, delay means connected tothe output of said thyratron means and to said relay means, a motorconnected to said output shaft, means operable by said relay means tocontrol the direction and speed of rotation of said motor whereby saidoutput shaft is caused to assume the same angular position as said inputshaft.

2. In a servo mechanism or the class described, a manually operableinput shaft. an output shaft operated by a two phase motor, means forgenerating an error voltage having an amplitude proportional to thedifference in angular position between said output shaft and said inputshaft, means for firing a thyratron at a predetermined point in itscycle depending on the amplitude of the error voltage, a relay connectedin the output circuit of said thyratron for controlling the direction ofrotation and speed of said motor and delay means connected to the outputcircuit of the said amplifier and to said relay whereby current ismaintained above a predator. mined minimum in said thyratron outputcircuit and said relay during a portion of the negative half cycle ofsaid thyratron.

8. In a servo mechanism of the class described, an input shaft, anoutput shaft, means for generating an error voltage the amplitude ofwhich is proportional to the difference in angular position between saidinput shaft and said output shaft, a two phase motor operativelyconnected to said output shaft through gearing means. means comprising asingle pole double throw relay connected to a source of power and tosaid V motor for alternately connecting said source of power directly toone of said motor windings and to the other of said winding in serieswith a capacity thereby alternately applying a clockwise andcounterclockwise torque to said motor, said relay being connected in theoutputof a single thyratron amplifier and operable thereby to itsalternate positions of contact means for impressing said error voltageon the input of said thyratron amplifier whereby said thyratron iscaused to fire at a predetermined point in its cycle depending on theamplitude of said error voltage and delay means connected in the outputcircuit of said amplifier and to said relay whereby current ismaintained above a predetermined minimum in said thyratron outputcircuit during a portion of the negative half cycle thereof.

4. In a servo mechanism of the class described.

' a single thyratron amplifier, an alternating ourrent power supplytherefor, means for impressing an alternating current bias on saidthyratron, means for superimposing on said alternating current bias anerror voltage which is proportional to the difference in angulardisplacement between an output shaft and an input shaft, relay meansactuated by the pulsating current output of said thyratron forconnecting the power supply directly to one field coil of a two phasemotor and to the other field coil in series with a condenser and forreversing said field and condenser connections in a predetermined timedrelation. a delay network connected to said thyratron output and to saidrelay means to maintain the current in said relay means above apredetermined minimum during a portion of the time when a negativepotential is impressed on said thyratron output from said alternatingcurrent source, damping means for said motor and gear reducing meansconnecting said motor to said output shaft and to I said error voltageproducing means, whereby said output shaft is maintained in correctanswrelatlonship with respect to said input shaft.

In a servo mechanism. of the class described, single thyratronamplifier, an alternating eunrent power supply therefor, meansimpressing; an alternating ciao on said. thyratron and means foradjusting; the phase and amplitude of said bias, means ror superimposingan error yoltage on said alternating current hias which a function ofthe difference in angular displace ment between an output shaft and aninput shaft. a two phase motor connected through gearing to said outputshaft and to said error voltage means, clamping means for said motor,means for operating said motor in accordance with the amplitude andphase of said error voltase comprising a re lay connected in the outputof said thyratron op erable to alternately apply a leading potential toone or the other of the field coils of said motor thereby successivelyapplying a clockwise and counter-clockwise torque thereto in timed relation to the frequency of the alternating current source, delay meansto maintain the current in said relay above a predetermined average fora portion or the negative half cycle of the potential applied to saidthyratron output circuit whereby the average torque applied to saidmotor over a period of several cycles is of such a mag- 'nitude anddirection as to correct any error oi angular displacement between theoutput shaft and input shaft. H V

6. In a servo mechanism or the class described a single thyratronamplifier. an alternat ng current power supply therefor. an inp shaft,an

-output shaft. electrical means associated with said "output shaft andsaid-input shaft and connected to the input of said thyratron amplifierfor producing an error voltage of a phase and'amplitucle proportional tothe angular displacement between said output and input shafts, analternating current bias means connected to the input of said thyratronand in series with said error voltage producing means, a condenser andvariable resistor associated with said alternating current bias supplymeans for adjusting the phase and amplitude of the bias potentialapplied to theinput of said thyratron, a single pole double throw relayhaving its relay coil connected in the output circuit of said thyratronamplifier and its fixed contacts to opposite ends of the field coils ofa two phase motor the remaining ends of which are connected together andto one terminal of an for producing a voltage which is a function of thedifference in angular position between said input shaft and said outputshaft, a single gas discharge tube actuated by said voltage, a delaycircuit connected to the output circuit of said gas discharge tube andrelay means operable by the output current of said gas discharge tubefor controlling the direction and amount of. rotation of the motorconnected to said otuput shaft whereby the difference in angularposition between said input and output shafts is minimized.

8. :In a device of the character described, a gas discharge tube havingan input circuit and an output circuit, means for supplying analternating current potential to said input circuit and said outputcircuit, means for adjusting the phase and amplitude of the alternatingcurrent potential supplied to said input-circuit, means forsuperimposing on said alternating current input potential asecondpotential having a phase and amplitude'which is a function of thedifference in angular position between an output shaft and an inputshaft, relay means having a single movable contact and two fixedcontacts connected in the output circuit of said gas discharge tube andoperable thereby to cause said movable contact to alternately engagesaid fixed contacts in timed relation to the pulsating current producedin said output circuit by the firing action of said gas discharge tube,delay means for maintaining the pulsating current in said output circuitabove a predetermined value dur ing a portion of the negative half cycleof potential applied to said gas discharge tube output circuit, motormeans electrically connected to said relay contacts and mechanicallyconnected to said output shaft, means including said relay foralternately applying a clockwise and counterclockwise torque to saidmotor in accordance with the alternate engagement of said movablecontact with said fixed contacts thereby causing said motor to rotatesaid output shaft in such a direction and to such an extent that saidoutput shaft assumes substantially the same angular position as saidinput shaft.

9. In a device of the character described, a gas discharge tube havingan input circuit and an 'output circuit, means for supplying analternating current potential to said input circuit and said outputcircuit, means for superimposing on said alternating current inputpotential a second potential having a phase and amplitude which is afunction of the difference in angular position between an output shaftand an input shaft, relay means having a single movable con tact and twofixed contacts connected in the output circuit of said gas dischargetube and operable thereby to cause said movable contact to alternatelyengage said fixed contacts in timed relation to the pulsating currentproduced in said output circuit by the firing action of said gasdischarge-tube, delay means for maintaining the pulsating current insaid output circuit above a predetermined value during a portion of thenegative half-cycle of potential applied to said gas discharge tubeoutput circuit motor means electrically connected to said relay contactsand mechanically connected to said output shaft, means including saidrelay for alternately applying-a clockwise and counter clockwise torqueto said 'i'notor in accordance with the alternate engagement of saidmovable contact with said fixed contacts thereby causing said motor torotate said output shaft in such a direction and to such an extent thatsaid output shaft assumes substantially the same angular position assaid input shaft. f

10. In a device'of the character described, a gas discharge tube havingan input circuit and an output circuit, means for supplying an altenating current potential to said input circuit and said output circuit,means for superimposing on said alternating current input potential asecond potential having a phase and amplitude which is a function of thedifierencein angular lay means and mechanically connected to said outputshaft, means including said relay for alternately supplying a positiveand negative torqu to said motor at a frequency corresponding to that ofthe alternating supply source thereby causing said motor to rotate saidoutput shaft in such a direction and to such an extent that said outputshaft assumes substantially the same angular position as said inputshaft.

11. In a device of the character described, a gas discharge tube havingan input circuit and an output circuit, means for supplying analternating current potential to said input circuit and said outputcircuit, means for adjusting the phase and amplitude of the alternatingcurrent potential supplied to said input circuit, means forsuperimposing on said alternating current input potential a secondpotential having a phase and amplitude which is a function of thedifierence in angular position between an input shaft and an outputshaft, relay means connected in the output circuit of said gas dischargetube and operable by the output current thereof, delay means formaintaining the current in said output circuit above a predeterminedvalue duringa portion of the half cycle when a negative potential isapplied to said gas discharge tube output circuit, motor meanselectrically connected .7 9 1 10 to said relay means and mechanicallyconnected to said output shaft, meam including said relay REFERENCES 0mmfor alternately supplyin a poflflve a d negative The followingreferences are of record in the torque to said motor at a frequencycorrespondfi of this patent: v ing to that of the alternating supplysource there- 5 m m by causing said motor to rotate said output shaltSTATES in such a direction and to such an extent that Number Name Datesaid ouput shalt assumes substantially the same 11551917 Richter 24,4925nn ulnr position u an! a t ut, 3 Murphy Jan- 1 19 m 011,333,; 10 1,8 ,04 Drake Dec. 29, 1931 1,960,350 Shackleton et al. May 29, 1934 2,089,914Freeman Aug. 10, 1987

